Lipids are molecules that consist of both hydrophobic and hydrophilic groups or regions within the same molecule. The ratio between the hydrophobic and hydrophilic portions of the molecule determines its physical properties in an aqueous environment. The uses of natural phospholipids as additives include, for example, food emulsifiers, cosmetics, industrial surfactants, and, and pharmaceutical drug-delivery systems. U.S. Pat. Nos. 4,086,257, 4,097,502, 4,097,503, 4,145,410 and 4,159,988 disclose various modifications of the polar-head-group region of natural phospholipids which lead to unique physical properties.
Various compounds prepared from lecithin have been described. For example, phosphatidyl polyethylene oxide compounds, their preparation and their use to encapsulate drugs in a drug-delivery system are mentioned in U.S. Pat. No. 4,426,330. Functionalized oxylalkylated lecithin and phosphatidyl-alkanolamine derivatives are described in U.S. Pat. Nos. 2,801,255, 3,542,820, 3,577,466 and 4,254,115.
Liposomes are microscopic spheres formed of thin durable lipid membranes. This membrane structure allows liposomes to regulate the passage of an entrapped drug into the bloodstream, a feature that offers potential for improving drug effectiveness and reducing side effects associated with certain drugs.
The emphasis of the bulk of liposome research is on injectable pharmaceutical products designed to improve the efficacy and reduce the toxicity of selected existing and new drugs used to treat cancer and infectious diseases. The use of liposomes and other lipid structures, such as micro-emulsions, micelles, and drug/lipid complexes, for drug delivery has been widely proposed. Such lipid structures, and particularly liposomes, have the potential for providing controlled release of an administered drug over an extended time period, and of reducing the side effects of the drug by limiting the concentration of free drug in the bloodstream. These advantages apply to a variety of routes of administration, including intravenous, intramuscular, and subcutaneous application to muscular tissue, or delivery by inhalation. Where liposomes are administered by intravenous delivery, liposomes provide a further advantage of altering the tissue distribution of the drug. Liposome drug delivery systems have been reviewed. See Gregoriadis, G., in Liposomes, vol III, Poznansky, M. L., et al, Pharm. Revs., 36(4):277 (1984).
In general, soon after entering the bloodstream, liposomes are recognized as foreign particles and removed from circulation by specialized cells residing primarily in the liver and spleen. These cells and organs are components of the reticuloendothelial system or "RES". Following uptake by the RES, the liposomes gradually release the drug in its free form back into the bloodstream. This RES uptake and release pattern is useful when the intent is to treat certain diseases which reside in the RES or to avoid a high concentration of the drug in the bloodstream. However, as a result of this RES-uptake mechanism, liposomes have not been shown to be as effective when it is desirable to target the drug to other parts of the body.
Several years ago it was recognized that liposomes capable of evading the RES-uptake mechanism might provide opportunities to develop new products with improved therapeutic profiles. Accordingly, research efforts were initiated to created methods for rendering liposomes delivered intravenously "invisible" to the RES, thereby increasing the circulation time of the liposome-entrapped drug in the bloodstream to improve the probability that drug-carrying liposomes will reach diseased tissues and organs.
One method for decreasing RES recognition of injected liposomes involves a specially synthesized polyethyleneglycol ("PEG") derivatized lipid, and the other involves a naturally occurring phosphatidylinositol ("PI") lipid. These PEG and PI molecules bind water molecules to the liposome surface. It is believed that this binding is the mechanism that disguises the liposomes from the RES and significantly increases blood circulation times as compared to both non-hydrophilized liposomes and the drug in its free form. See U.S. Pat. Nos. 4,426,330 and 4,534,899.
Hydrophilized liposomes may also provide significant improvements in the efficacy of certain cancer drugs, antibiotics and other therapeutics, while decreasing exposure of normal tissues to such drugs, thereby reducing some of the side effects associated with conventional therapies.
In the case of liposomes, optional size for use in parenteral administration is generally between about 100 nm and 300 nm. Liposomes in this size range can be sterilized by passage through conventional filters having a particle size discrimination of about 200 nm. This size range of liposomes also may favor biodistribution in certain target organs, such as liver, spleen and bone marrow, and gives more uniform and predictable drug-release rates and stability in the bloodstream. See, e.g., A. Gabizon, et al, J. Liposome Research 1:123 (1988). Liposomes whose sizes are less than about 300 nm also show less tendency to agglutinate on storage, and are thus generally safer and less toxic in parenteral use than larger size liposomes.
It may also be desirable to prepare uniform size liposomes in a selected size range less than about 100 nm. For example, small unilamellar vesicles (SUVs) having sizes between about 30-80 nm are useful in targeting to tumor tissue or to hepatocyte cells, because of their ability to penetrate the endothelial lining of capillaries. SUVs are also advantageous in opthamalic liposome formulations, because of the greater optical clarity of the smaller liposomes.
Sonication, or ultrasonic irradiation, is a known method that is used for reducing liposome sizes by shearing and especially for preparing SUVs. The processing capacity of this method is quite limited, however, since long-term sonication of relatively small volumes is required. Also, localized heat build-up during sonication can lead to peroxidative damage to the lipids, and sonic probes shed titanium particles which are potentially quite toxic in vivo.
A third general size-processing method known in the prior art is based on liposome extrusion through uniform pore-size polycarbonate membranes (F. Szoka, Jr., et al, Proc. Natl. Acad. Sci. USA 75:4194(1978)). This procedure has advantages over homogenization and sonication methods in that several membrane pore sizes are available for producing liposomes in different selected size ranges. In addition, the size distribution of the liposomes can be made quite narrow, particularly by cycling the material through the selected-size filter several times. Nonetheless, the membrane extrusion method has limitations in large-scale processing including problems of membrane clogging, membrane fragility, and relatively slow throughput.
U.S. Pat. No. 4,737,323 describes a liposome sizing method in which heterogeneous-size liposomes are sized by extrusion through an symmetric ceramic filter. This method allows greater throughput rates, and avoids problems of clogging since high extrusion pressure and reverse-direction flow can be employed. However, like the membrane extrusion method, the filter-extrusion method requires post-liposome formation sizing. Further, the method may be limited where uniform-size SUVs are desired.
One limitation of all of the above-mentioned methods in the loss of encapsulated material as large liposomes are broken and reformed as smaller vesicles. Furthermore, in none of the liposome-preparation methods mentioned above are liposomes with a narrow, substantially symmetrical size distribution produced, nor are liposomes produced with small enough uniform size to cross transcellular barriers. Finally, liposomes are known to be generally unstable and always exhibit uncontrolled leakage of drugs prior to reaching the delivery cite.
More recently, there have been investigations regarding the potential application of lipid-coated iron oxide particles as magnetic resonance contrast agents for imaging inflammatory processes. It has been shown, for example, that intravenous injections of lipid-coated iron oxide particles can serve as contrast agents and the effects on proton relaxation times have been reported. See, e.g. MR Imaging of Absessess by Use of Lipid-coated Iron Oxide Particles, Radiological Society of North America, 76th Annual Meeting, November (1990). In European Pat. No. 272,091 there is described an in vivo delivery vehicle which comprises as the delivery vehicle a superparamagnetic and ferromagnetic particle 20-10,000 nm in diameter, an amphiphilic material associated with said particles to form what is described as an amphiphilic-associated substrate, and an encapsulating layer including at least one such layer associated with the amphiphilic substrate, the outer of the encapsulating layers being a bio-compatible encapsulating layer. The specific targeting or delivery of the compound to particular tissues, organs or cells is reported, as well as extended circulation and serum stability.
It is therefore a general object of the invention to provide a novel uniform size inorganic core liposome composition of uniform size which solves or substantially overcomes problems associated with the prior art. It is a more specific object of this invention to provide a liposome with an inorganic core of substantially uniform sub 100 nm size, shape, charge and chemistry which can be used for invivo and invitro medical applications (e.g., the administering of a drug via the bloodstream), and a process for making them. Another object of the invention is to provide a method of preparing a uniform size inorganic liposome composition without requiring post-liposome formation extrusion or other sizing procedures. Still another object of the invention is to provide such a method which can be practiced to achieve relatively high encapsulation rates, and in which loss of non-encapsulated material is avoided.
It is also an object of the invention to provide a novel phenyl lipid composition with enhanced circulation time in the bloodstream, and to the method of preparation and to the use of such compounds, particularly in solubilizing, in an aqueous environment, water-insoluble compounds, and to the use of such phenyl lipid compounds for modifying the solubility of the uniform size inorganic core liposome composition.